The present invention relates to an improvement in a method of and an apparatus for heat-treatment of welded pipe having a reinforcement weld bead therealong by heating the pipe uniformly by using an induction heating device and then by cooling it.
In producing large-diameter steel pipe by a process such as the so-called UO process, it is necessary to apply a heat-treatment to the pipe which has been welded at the abutted edges of the sheet which has been formed into the shape of the pipe in order to secure desired properties of the pipe. As a heating device for such heat-treatment, there is utilized an induction heating device because of its rapid heating capacity and high operating efficiency. However, just after the heating of the welded pipe by an induction heating device for carrying out the heat-treatment, the temperature of the bead of the pipe is generally lower than that of the metal of the pipe itself. FIGS. 1a and 1b show the temperature distribution of the bead and the area around the bead. The following are known facts about such temperature distribution: if the thickness of the bead, and hence the reinforcement rate (2 .DELTA. t/t) is great, the temperature distribution appears as shown in FIG. 1a; and if the rate is small, the temperature distribution appears as shown in FIG. 1b. The maximum temperature difference .DELTA. T max is, say, 100.degree. to 200.degree. C. Any great temperature difference between the base metal and the bead affects the quality of the treated pipe adversely, making it necessary to control such difference within an allowable limit for securing the desired quality of the treated pipe.
The allowable limit for the temperature difference varies according to heat-treatment methods; one example being .+-.10.degree. C. for treatment at 950.degree. C.
As a means for making the temperature difference between the pipe metal and the bead smaller, there is conventionally utilized a longer time for heating the welded pipe, during which time heat conduction in the pipe makes the difference smaller.
However, as shown in FIG. 2, the layout of the conventional heat-treatment apparatus using an induction heating device, has feed rollers 1 and 2, an induction heating coil 3 and a cooling device 4 for continuous heat-treatment of the welded pipe. In such a treatment line, however, there is the limit on the distance between feed rollers depending on the length of the treated pipe (in most cases, 6-20 m) and also on the distance between the heating coil 3 and the cooling device 4 depending on the heat-treatment methods and the efficiencies thereof. The length of the heating device itself is restricted. Therefore, the means to make the temperature difference smaller merely by lengthening the heating time may sometimes be impracticable under various restrictive conditions.
Another means for making the temperature difference smaller is preheating of the bead before the heating of the pipe for heat-treatment, by using such devices as a gas torch or induction bead heater, such as disclosed in U.S. Pat. No. 3,804,390. However, as a result of the experiments conducted by the inventors of the present invention, such means has the following drawbacks:
(a) The heat used for pre-heating the bead before heating the whole of the pipe by the induction heating coil diffuses in the direction of the pipe circumference while it passes the cooling device after the induction heating coil, making the temperature of the part of the pipe metal adjacent to the bead higher than that of the bead itself, resulting in excessive heating of said part of the pipe metal. Therefore, as the pipe becomes cool and contracts, deformation is concentrated at said part which has been excessively heated, the thus produced deformation being much greater than in other parts, degrading the resulting pipe with respect to roundness of the cross-section of the pipe by as much as 20 to 30 mm in a diameter of 700 mm, as compared with 15 mm when the bead is not pre-heated.
(b) In general, the higher the frequency of the current supplied to the induction heating coil, the higher the heat concentration but the shallower the depth of penetration of the heat. Also, the lower the temperature of the heated object, the shallower the depth of penetration of the heat. Thus, in the method comprising pre-heating of the bead before heating the whole of the pipe, uniform heating only of the bead in the direction of its thickness, cannot be attained if a high frequency current is used, in view of the relationship with said heat concentration and the depth of penetration of the heat. As a result, the frequency appropriate for the pipe wall thickness of 10 mm is 50 to 150 Hz.
(c) Heat control can be carried out only with difficulty because of the long distance between the position for controlling and that of cooling.
(d) Not only the bead but also the area around the bead is heated, such widened area of heating reducing heat efficiency.